In continuous ink jet printing, ink is supplied under pressure to a manifold region that distributes the ink to a plurality of orifices, typically arranged in a linear array(s). The ink discharges from the orifices in filaments which break into droplet streams. The approach for printing with these droplet streams is to selectively charge and deflect certain drops from their normal trajectories. Graphic reproduction is accomplished by selectively charging and deflecting drops from the drop streams and depositing at least some of the drops on a print receiving medium while other of the drops strike a drop catcher device. The continuous stream ink jet printing process is described, for example, in U.S. Pat. Nos. 4,255,754; 4,698,123 and 4,751,517, the disclosures of each of which are totally incorporated herein by reference.
Achieving improvement in permanence and darkness of ink jet print remains a top priority in the printing business. For ink jet business to grow in the non-porous and semi-porous substrate arts, such as printing on metals and plastics, significant improvement in water resistance, print quality, and print adhesion of the image must be demonstrated.
Dye-based aqueous inks have not been suitable for printing on metals or plastics, due to their inherent high surface tension and their inability to wet non-porous substrates. Even when the surface tension is lowered enough to improve wetting, the ink does not stick to the surface, due to lack of surface penetration. Non-porous and semi-porous substrates have been resistant to dye-based aqueous inks because the print image is not suitably waterfast, and is not resistant to smudging or bleeding. Only solvent inks can produce rub-fast images on these surfaces, but with the inherent disadvantage of high volatile organic compounds, safety and health concerns.
It is seen then that there is a need for an improved method of optimizing waterfastness and minimizing smudging and bleeding of environmentally safe aqueous, black or non-black, dye-based inks, when printed on non-porous or semi-porous substrates.